The referenced U.S. Pat. No. 3,978,006 describes a gas sensor which has an ion conductive body on which an electrode layer is applied. This electrode layer is a mixture of a ceramic material and an electron conductive material, applied to the surface of the sensor. The layer of ceramic-ion conductive material and the electrolyte body are then, together, sintered. This method results in sensors eminently suitable to determine whether exhaust gases from internal combustion engines are reducing or oxidizing. When carrying out this process, particularly under mass production conditions, it is desirable to sinter the ceramic material applied to the surface of the sensor body together with sintering of the sensor body itself. The same ceramic material and the same sintering conditions are used. It has been found that sensors made in accordance with this process have a comparatively high electrode polarization, since the cermet electrodes which will result--that is, the ceramic-metal mixtures which form the electron conductive layer--will sinter to substantial density.
The sensors which indicate change in the gas composition as a voltage change can be loaded only lightly, that is, only small loading currents can be applied thereto without masking of the sensor signal itself. This requires, however, a complex sensing electronic network which is connected to the sensor itself in order to be able to distinguish between sensor signals and their characteristics, in dependence on the oxygen content of the gases to which the sensor is exposed. Such sensors are operative only at relatively high temperatures, since the internal resistance of the solid electrolyte body, due to the temperature dependence of the ion conduction, rises rapidly with decreasing temperature. The response sensitivity of such sensors is relatively low, that is, the time of reaction of sensor output with respect to change in the exhaust gas composition is relatively long, since the diffusion of gases through the densely sintered cermet electrode to the solid electrolyte body itself is impeded by the dense composition of the electrode.
Sensors which operate on the basis of current limits, and which indicate a change in the composition of the gases by change of the diffusion limiting current, have been proposed in which an artificial barrier, in form of a diffusion layer, is provided. Such sensors on the basis of current limits must have electrodes which can be loaded efficiently. The diffusion layer must have an acceptable low diffusion resistance in order to obtain current limit or threshold values which are unambiguously dependent on gas concentration. Diffusion layers with desirable diffusion resistance can be reproduced only with difficulty, particularly under mass production commercial conditions, and additionally reduce the sensitivity of the current limiting sensor and also increase its response time.